Dan Jones

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                DISP * RPM * VE
        CFM =   ---------------
                     3456 

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Selecting the Proper Size Carb by Dan Jones
There are a number of sizing formulas that can be used to estimate the best
flow rating for a given application.  The standard equation relating engine
size and required carb flow is:

where:
        DISP    = engine displacement in cubic inches
        CFM     = required carb flow in cubic feet per minute
        RPM     = maximum engine speed in revolutions per minute
        VE      = volumetric efficiency (dimensionless, 1.0 = 100%)
        1728    = conversion factor between cubic inches and cubic feet 
                = 12*12*12
        2       = conversion factor for four stroke engine
This equation can be simplified to:
                DISP * RPM * VE
        CFM =   ---------------
                     3456
Note this sizing formula is simply a relationship between cylinder volume and 
the flow required to fill that volume at a given engine speed.  Also note, 
for a four stroke engine, displacement is divided by two because an intake
stroke occurs every other revolution.  Another implicit assumption is that 
the carb is mounted on a plenum style intake.  Independent runner intakes will
require much different sizing.  While it's easy to determine displacement and
maximum rpm, you'll probably have to guess at the third variable, volumetric 
efficiency (VE), unless you have access to a dyno.  Volumetric efficiency is
a simply a measure of how efficiently an engine fills its cylinders.  A stock, 
low performance, street engine may have a VE between 0.7 and 0.8.  High
performance street engines may fall between 0.8 and 1.0, while highly tuned
race engines can have VE's exceeding 1.0, perhaps as high as 1.25.
One other thing to understand when using the formula above is that a carb will 
only flow in the presence of a pressure differential.  On one side of the carb 
there is atmospheric pressure and on the other side is manifold pressure
(usually referred to as manifold vacuum since it is typically lower than
atmospheric pressure).  Since engines vary in their manifold vacuum
characteristics, a standardized pressure differential was established to 
provide a meaningful comparison for different carbs.  Before this standard, 
venturi size was used for comparison.  The standard for four barrel carbs is
a pressure differential equal to 1.5 inches of mercury (Hg).  What this means
is a 4 barrel carb rated at 500 CFM will flow 500 CFM of air, at wide open 
throttle, when a pressure differential of 1.5 In Hg is applied across it.
When installed on an engine, this same carb may flow more or less.  Two barrel
carbs are usually rated at a different pressure differential (3.0 In Hg).  The
reason for this is primarily historical.  When 4 barrel carbs first came into
popular use, the vacuum pumps used to rate 2 barrel carbs were unable to pull 
the same pressure differential across a 4 barrel carb, so 4 barrels were rated 
at a lower pressure drop.
Flow ratings from one standard can be related to flow ratings from another
standard.  For 2 and 4 barrel carbs:
        Flow @ 1.5 In Hg = CFM Rating @ 3.0 In Hg 
                           ----------------------
                                SQRT(3.0/1.5)
Which is approximately:
        Flow @ 1.5 In Hg = CFM Rating @ 3.0 In Hg 
                           ----------------------
                                   1.414
This relationship is derived from the fact that, for incompressible flow, the 
volumetric flow rate through a venturi is proportional to the square root of
the pressure differential:
        Q = K1*A2*SQRT(2*Gc/Rho)*SQRT(P1-P2)
or more simply:
        Q = K2*SQRT(P1-P2)
where:
        Q       = volumetric flow rate
        K1      = flow coefficient
        A2      = downstream area of the venturi
        Gc      = gravitational constant
        Rho     = density
        P1      = inlet pressure
        P2      = pressure at venturi minimum area
        K2      = K1*A2*SQRT(2*Gc/Rho)
Computing the relationship for volumetric flow rate at the two flow 
differentials and equating yields the conversion formula.  Note the implicit 
assumption that the flow coefficient does not change (it can).
As an example of using the sizing formula, let's say we have a modified 4.1
liter (252 cubic inches) Buick V6 with a VE of 0.9 and we plan to turn no
more than 6400 rpm.  Plugging our numbers into the formula yields a 
theoretical estimate of:
              252 * 6400 * 0.9
        CFM = ----------------
                    3456
   
            = 420 CFM
In practice, Joe Murawski of the Wedge list runs a 4.1L Buick in his Triumph
TR7 and has tried a variety of carbs, in sizes ranging from a Holley 390 to
a 785 CFM Quadrajet, settling on a 500 CFM Edelbrock/Carter AFB as providing
the best power and driveability.  His carb choice is somewhat larger than that
predicted.  For reasons discussed below, we'll see this is not unusual.
While the formula above may yield useful estimates, it is not necessarily the 
ideal it is often portrayed to be.  If you have a carb that can flow 500 CFM
in the same application and still properly atomize the fuel, it should make
more power than a 400 CFM carb.  From this perspective, larger is better.
Ideally, a carb would present zero restriction to the intake stroke.  Such a 
carb would have an infinite flow rating.  Unfortunately, carbs require a 
pressure differential to properly mix fuel with air, which is why carb sizing 
is important (and why the above formula is useful).  Keep increasing the size
of a carb and, at some point, the booster venturis will not properly atomize
the fuel/air mixture and droplets of liquid fuel will be pulled into the
cylinders.  Not only is this bad for performance, it's also hard on the engine.  
The liquid fuel tends to wash oil off the cylinder walls, increasing ring
and bore wear.  This is a particular problem with engines using large overlap
cams, since they provide lower vacuum levels.  When using a larger carb and 
cam, proper tuning (carb and ignition) becomes more important.
It's important to understand the basic sizing formula is just a guideline.
It ignores a number of important factors such as manifold design, cam timing,
vehicle weight, gearing, transmission type, intended usage, etc. Furthermore, 
it ignores important differences in carb design like venturi efficiency, bore 
layout, and secondary style and method of actuation.  In practice, I have found 
that the above formula applies mainly to square bore carbs with non-air valve 
secondaries (Holleys, Autolites), and even then it can be conservative for a
performance application.  It typically yields a compromise of fuel efficiency 
and power.
Using a dual plane, divided plenum, intake usually allows the use of a carb
with a larger CFM rating than with a single plane, open plenum, intake.  This 
is because the divider cuts the effective plenum volume in half, increasing
the signal to the boosters.  Because of this fact, Edelbrock suggests
multiplying the CFM predicted by the basic sizing formula by 1.1 to 1.3 for 
single plane manifolds and by 1.2 to 1.5 for dual planes.
As another example, consider the engine I'm currently running in my Pantera.
It's a 351C Ford with Aussie 2V quench heads, 1 3/4" headers, and a single
plane, open plenum, Weiand Xcelerator intake manifold.  Since I retained the
stock cast pistons, I chose a cam with a shift point of 6000 rpm.  As a
guess, pick 0.9 for the VE.  Since the Pantera is relatively light with short
gearing, pick the high side of the range for K (the intake factor):
              K*DISP * RPM * VE   1.3*351*6000*0.9
        CFM = ----------------- = ----------------
                    3456               3456
            = 713 CFM
This agrees with real world Pantera club experience with Holleys on street
modified 351C's.  600 CFM carbs provide good throttle response and fuel economy 
but give up 20+ peak horsepower to 750 carbs.  On the downside, the 750 hurts 
fuel economy and has poorer throttle response.  The happy medium is probably
somewhere in between.  Note we're referring to stock Holley carbs here, not
custom models with milled choke horns, thinned butterflies, and improved 
booster designs.  Those modified carbs can flow more mixture, while providing 
adequate atomization. 
I chose a Holley 735 from a 428CJ application which seems to work well.  This 
carb, while flowing nearly as much as a 750, has a venturi cluster design that 
provides a stronger signal.  Throttle response and fuel economy are relatively 
good (20+ mpg on the highway), without incurring a noticeable power penalty.
Two other important considerations are bore layout and method of secondary
actuation.  Carbs with air valve secondaries (Carters and Rochester Quadrajets), especially those with spread bore layouts (Thermo Quads, Quadrajets), can 
usually be sized larger than square bore Holleys and Autolites.  This is 
because the smaller primaries increase the flow speed through the boosters, 
providing better atomization, while the air valve secondaries passively 
restrict air flow until the engine can handle it.  Taking these two factors 
into consideration, Vizard suggests the following two rules of thumb for street performance engines where power is more important than fuel economy.  For air 
valve secondary carbs with an upper rpm limit of 6000 rpm, use:
        CFM = 2.3 * DISP
For square bore non-air valve secondary carbs use:
        CFM = 2.0 * DISP
For engine speeds above 6000 rpm, multiply by the ratio of maximum rpm to
6000 rpm.  Note the second formula yields 702 CFM for my 351C example, which 
is close to the basic sizing formula with the intake manifold correction 
factor applied.
As an extreme example, I've successfully used a 750 CFM Quadrajet on a
relatively stock 231 cubic inch Buick V6.  With the Qjet, it got slightly
better fuel economy than the previous 2 barrel carb (due to the small
primaries) and had noticeably more power (due to the huge secondaries).  The
driveability of the carb was fine with no bogs or flat spots.  On the V6, I'm 
sure it never pulled anywhere near the 750 CFM rating but it did pull what
it required.  You could never put a Holley 750 on a little low compression V6
and expect to make it work.  The air valve secondaries allow the use of much
larger CFM ratings without incurring driveability problems.  There is a price 
to be paid however.  Even when wide open, air valve secondaries are slightly 
more restrictive to airflow than non-air valve secondaries.  
While these formulas should help you choose a carb flow rating, nothing beats 
trail and error.  Also, once you have a carb installed, you can determine how 
restrictive it is by using a vacuum gauge to measure the difference between
atmospheric pressure and the pressure under the carb.  With the air cleaner
removed, the air above the carb will be essentially atmospheric.  If there's
any difference between it and the pressure sensed under the carb, it's due
to the carb.  The higher the difference, the greater the restriction. 
Measurements should be made at wide open throttle and 0.7 inches of mercury
is considered non-restrictive. 
Dan Jones 

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I fixed it for you.